The Effects of MgO and Al2O3 Content in Sinter on the Softening–Melting Properties of Mixed Ferrous Burden

The softening–melting properties of mixed ferrous burden made from high-basicity sinter with increased MgO and Al2O3 content and acid pellets was investigated for optimization. The influences of MgO and Al2O3 are discussed with the aid of phase analysis. The results showed that, with decreasing MgO mass%/Al2O3 mass% in mixed burden, all the softening–melting characteristic temperatures decreased, which can be attributed to the low melting temperature and viscosity of the slag caused by MgO and Al2O3. The permeability of the melting zone deteriorated again when MgO mass%/Al2O3 mass% decreased to a certain content. The softening interval widened slightly at first and then narrowed, while the melting interval first increased slightly and then increased greatly later. It can be deduced that the softening properties were improved, but the melting properties were worsened. Under comprehensive consideration of its softening–melting properties, permeability, iron ore reduction and the thermal state of the blast furnace hearth, the optimal softening–melting properties of a mixed ferrous burden with MgO mass%/Al2O3 mass% of 0.82 is optimal.


Introduction
The blast furnace (BF)-oxygen steelmaking route which uses iron ores as the raw ferrous materials is the primary source for worldwide steel production. The BF is the biggest counter-current reactor involving downward iron ores (sinter, pellets and lump ore) and upward gas flow for producing pig iron [1]. Normally, the ferrous burden and coke are charged into the BF separately, giving the furnace stack a layered structure. The ore layer undergoes a heating and reduction process in the BF. On reaching the lower part of the BF at much higher temperatures, they soften and melt, forming the region known as the cohesive zone. The softening and melting (SM) properties of ferrous burden have a signification effect on the shape, position and thickness of the cohesive zone [2,3], determining the gas utilization and heat transfer, which vitally influences BF operational performance and productivity [4].
A sinter with high basicity has good reduction and SM properties while its Fe content is low. A high Fe content and a low gangue content are favorable for acid pellets. However, it has inferior reduction and SM properties due to low-melting fayalitic slag [5]. The pelletizing process is much cleaner than the sintering process, while the former has low yields [6]. Sinter and pellets have become the popular ferrous materials for BF ironmaking [7]. It is a feasible direction to use mixed ferrous burden made with high-basicity sinter and acid pellets for green metallurgy due to their complementary advantages [8].
Recently, because of the increasing scarcity of good-quality iron ores, high Al 2 O 3 iron ores of low grade and low cost have been utilized to make high-basicity sinter, pursuing economic benefit [9,10]. Increased Al 2 O 3 content in BF ferrous burden leads to high viscosity [11][12][13][14] and bad desulfurization ability [15] of the slag and low permeability [16] in the cohesive zone. A popular countermeasure is adding MgO into sinter [17]. So, both the MgO and Al 2 O 3 content in sinter increase and the MgO mass%/Al 2 O 3 mass% changes, which are bound to influence the SM properties of high-basicity sinter and its mixed ferrous burden with acid pellets [18], consequently attracting our attention.
The effect of MgO or Al 2 O 3 content on the SM properties of sinter and pellets has been reported in many studies in the literature. The works of Guo et al. [19,20] demonstrated that the softening temperatures and the maximum pressure drop increased, and the melting temperature interval became wide with the increasing MgO content in sinter, because the MgO existed as a wüstite solid solution when sinter softened, and then on melting it brought a high-melting point slag. Li et al. [21] conducted a study of the effect of MgO content on the SM performance of high-gangue sinter and partially supported Guo's viewpoint. They believed that MgO content did not always improve the SM performance of sinter but it had a certain content limit. Wang et al. [22] proposed that high Al 2 O 3 content sinter was associated with a thicker melting-dripping zone, leading to the deterioration of sinter bed permeability. Our previous work [17] on the effects of MgO and Al 2 O 3 on the SM properties of high-basicity sinter found that MgO and Al 2 O 3 led to increased and decreased SM characteristic temperatures, respectively. The influence of MgO on SM properties was insignificant compared with Al 2 O 3 , which was attributed to the fact that MgO existed in wüstite as an FeO-MgO solid solution and Al 2 O 3 was distributed in slag. In the case of pellets, Iljana et al. [23] carried out experiments on metallurgical properties with different-sized pellets. They observed that the higher proportions of MgO in the silicate slag and in the wüstite resulted in increased softening temperatures. They also verified that increasing the MgO content can improve the softening properties of pellets via computational thermodynamics [24] and proposed the separation of Al 2 O 3 from iron ore concentrate to avoid deteriorating the softening properties. Chen et al. [25] reviewed the effect of MgO on the metallurgical properties of pellets, and pointed out that the softening properties were improved obviously, but the melting properties were not improved with increasing MgO content.
However, research on the SM properties of mixed burden composed of sinter and pellet is relatively limited compared to single ore species [26,27], especially regarding the comprehensive effect of MgO and Al 2 O 3 under the current conditions that both MgO and Al 2 O 3 in high-basicity sinter are increased. There are still some difficulties in adjusting the MgO and Al 2 O 3 content in sinter for the purpose of optimizing the SM properties of mixed ferrous burden. Therefore, the SM properties of mixed ferrous burden made from high-basicity sinter with increased MgO and Al 2 O 3 content and acid pellets were investigated in this study. The influence mechanisms of MgO and Al 2 O 3 were analyzed with the aid of phase analysis. We hope that the results will be useful to the establishment of a picture about the SM behavior of mixed burden, and its effect on BF operations.

Materials
Three kinds of high-basicity sinters and one kind of acid pellet, derived from actual ironmaking plants, were selected as experimental materials. The chemical compositions are listed in Table 1. It shows that there was only a little variation in the degree of the TFe content, FeO content and binary basicity (C/S) in the sinter samples. The changes in MgO and Al 2 O 3 content in the sinters were more interesting. From S 1 to S 3 , the MgO content increased from 1.70 to 3.16 mass% with increasing Al 2 O 3 content (from 1.14 to 4.03 mass%), while the MgO mass%/Al 2 O 3 mass% (M/A) decreased from 1.49 to 0.78.
The experimental samples were three kinds of mixed burdens. All the mixed burdens consisted of 70 mass% sinter and 30 mass% pellets, which is a general burden structure in China. The chemical compositions of mixed burdens are shown in Table 2. From S 1 P to S 3 P, MgO and Al 2 O 3 content increased continually, from 1.33 to 2.36 mass% and from 1.23 to 3.25 mass %, respectively. The M/A kept a decreasing trend from 1.08 to 0.73.

Experimental Procedure
The experimental equipment of the SM test is shown in Figure 1. A packed bed comprising mixed burden (200 g) packed in between two coke layers (20 mm each) was heated with a load of 0.5 kg·cm −2 in a graphite crucible. The particle sizes of iron ores and cokes were 10~12.5 mm and 6~8 mm, respectively. All the materials were dried at 105 • C for 24 h before testing. The dripping holes in the bottom of the crucible allowed the gas flow to pass through the burden and the molten iron/slag to drip. The mixed burden was heated at 10 • C·min −1 below 950 • C, and kept at 950 • C for 1 h, and then heated at 5 • C·min −1 over 950 • C. The gas flow was changed from N 2 (5 L·min −1 ) to a reducing gas composed of CO and N 2 with the ratio 30/70 (12 L·min −1 ) until the sample temperature reached 400 • C. A more detailed description of the test procedure was given in our previous work [28]. The experimental samples were three kinds of mixed burdens. All the mixed burdens consisted of 70 mass% sinter and 30 mass% pellets, which is a general burden structure in China. The chemical compositions of mixed burdens are shown in Table 2. From S1P to S3P, MgO and Al2O3 content increased continually, from 1.33 to 2.36 mass% and from 1.23 to 3.25 mass %, respectively. The M/A kept a decreasing trend from 1.08 to 0.73.

Experimental Procedure
The experimental equipment of the SM test is shown in Figure 1. A packed bed comprising mixed burden (200 g) packed in between two coke layers (20 mm each) was heated with a load of 0.5 kg·cm −2 in a graphite crucible. The particle sizes of iron ores and cokes were 10~12.5 mm and 6~8 mm, respectively. All the materials were dried at 105 °C for 24 h before testing. The dripping holes in the bo om of the crucible allowed the gas flow to pass through the burden and the molten iron/slag to drip. The mixed burden was heated at 10 °C·min −1 below 950 °C, and kept at 950 °C for 1 h, and then heated at 5 °C·min −1 over 950 °C. The gas flow was changed from N2 (5 L·min −1 ) to a reducing gas composed of CO and N2 with the ratio 30/70 (12 L·min −1 ) until the sample temperature reached 400 °C. A more detailed description of the test procedure was given in our previous work [28]. In the SM process, the ferrous burden bed deformed (softened and melted) and then hindered the gas flow. During the whole experimental process, several important In the SM process, the ferrous burden bed deformed (softened and melted) and then hindered the gas flow. During the whole experimental process, several important parameters such as the sample temperature, shrinkage ratio (∆h) and pressure drop (∆P) were continuously recorded by the thermocouple, the displacement meter and the pressure gauge, respectively. When the test was finished, the dripped substance and residual materials in the graphite crucibles were collected for further analysis.

Results and Discussion
A typical set of SM curves of the mixed burden is shown in Figure 2, which includes the shrinkage ratio (∆h) and pressure drop (∆P) changes against the sample temperature. To represent and compare the SM properties of these samples, some indexes were adopted. As evident from Figure 2, when ∆h reached 10% and 40%, the sample temperatures T 10% and T 40% were defined as softening start temperature and softening end temperature, respectively. The melting start temperature T m was the temperature that ∆P began to steeply increase; and T f was the melting end temperature at which ∆h tended to be unchanged. The softening temperature interval (T 40% -T 10% ) represented the softening zone while the melting temperature interval (T f -T m ) represented the melting zone. Because ∆P was at the low level in the softening range, the softening zone was expected to be wide while the melting zone should be narrow, indicating good SM properties of the burden. During the melting temperature interval, when the first dripping phenomenon was observed, the sample temperature was recorded as the dripping temperature T d . ∆P max was the maximum value of the pressure drop curve. S was defined as the permeability index of the melting zone and supposed to be the integral value of the pressure drop function over the melting zone (from T m to T f ), seeing the shaded area in Figure 2. The higher the value of S, the worse the permeability of the mixed burden in the melting zone.
parameters such as the sample temperature, shrinkage ratio (Δh) and pressure drop (ΔP) were continuously recorded by the thermocouple, the displacement meter and the pressure gauge, respectively. When the test was finished, the dripped substance and residual materials in the graphite crucibles were collected for further analysis.

Results and Discussion
A typical set of SM curves of the mixed burden is shown in Figure 2, which includes the shrinkage ratio (Δh) and pressure drop (ΔP) changes against the sample temperature. To represent and compare the SM properties of these samples, some indexes were adopted. As evident from Figure 2, when Δh reached 10% and 40%, the sample temperatures T10% and T40% were defined as softening start temperature and softening end temperature, respectively. The melting start temperature Tm was the temperature that ΔP began to steeply increase; and Tf was the melting end temperature at which Δh tended to be unchanged. The softening temperature interval (T40%-T10%) represented the softening zone while the melting temperature interval (Tf-Tm) represented the melting zone. Because ΔP was at the low level in the softening range, the softening zone was expected to be wide while the melting zone should be narrow, indicating good SM properties of the burden. During the melting temperature interval, when the first dripping phenomenon was observed, the sample temperature was recorded as the dripping temperature Td. ΔPmax was the maximum value of the pressure drop curve. S was defined as the permeability index of the melting zone and supposed to be the integral value of the pressure drop function over the melting zone (from Tm to Tf), seeing the shaded area in Figure 2. The higher the value of S, the worse the permeability of the mixed burden in the melting zone.  Figure 3 shows the effects of MgO and Al2O3 content in high-basicity sinter on the softening characteristic temperatures of mixed burdens. As the MgO and Al2O3 content increased and the M/A decreased, the softening start temperature T10% and the softening end temperature T40% showed decreasing trends from 1316 °C to 1238 °C and from 1453 °C to 1354 °C, respectively. The decrease extent of T10% from S1P to S2P was more significant than that observed from S2P to S3P. The softening interval (T40%-T10%) widened slightly from 137 °C to 149 °C at first, and then narrowed down to 116 °C. In terms of the position and thickness of the cohesive zone in BF, the upper edge of the cohesive zone and the corresponding T10% moved upward. The experimental softening results showed that the M/A of mixed burden had a certain content limit. The possible thinning of the thickness  Figure 3 shows the effects of MgO and Al 2 O 3 content in high-basicity sinter on the softening characteristic temperatures of mixed burdens. As the MgO and Al 2 O 3 content increased and the M/A decreased, the softening start temperature T 10% and the softening end temperature T 40% showed decreasing trends from 1316 • C to 1238 • C and from 1453 • C to 1354 • C, respectively. The decrease extent of T 10% from S 1 P to S 2 P was more significant than that observed from S 2 P to S 3 P. The softening interval (T 40% -T 10% ) widened slightly from 137 • C to 149 • C at first, and then narrowed down to 116 • C. In terms of the position and thickness of the cohesive zone in BF, the upper edge of the cohesive zone and the corresponding T 10% moved upward. The experimental softening results showed that the M/A of mixed burden had a certain content limit. The possible thinning of the thickness of the softening zone was achieved, which indicated that the softening properties of mixed ferrous burden were improved when adjusting the M/A. on softening was likely to be more dominant than MgO [17]. Thus, the softening temperatures were expected to decrease. It is indispensable to optimize the softening properties of mixed burden, but only when considering both the reduction property and gas permeability. In the softening process of the samples, its pressure drop was relatively low before intensely climbing. The wide softening zone benefited indirect reduction and decreasing coke rate. Therefore, it is believed that the softening property of S2P was the best where the certain M/A of the mixed burden was 0.82.

The Effects of MgO and Al2O3 on Melting Properties
The effects of MgO and Al2O3 content in sinter on the melting characteristic temperatures of mixed burdens are shown in Figure 4. With the increase in MgO and Al2O3 content in sinter, the melting start temperature Tm and the melting end temperature Tf decreased from 1490 °C to 1436 °C and from 1558 °C to 1531 °C, respectively. The melting interval (Tf-Tm) first increased slightly, and then increased greatly later. According to the experimental melting results, the lower edge of the cohesive zone and the corresponding Tf moved upward and the thickness of melting zone and corresponding Tf-Tm increased. It should be noted that once the M/A of the mixed burden was less than 0.82, the melting properties were worsened significantly, which was not the expectation. The dripping temperature Td of S1P, S2P and S3P was 1534 °C, 1506 °C and 1481 °C, respectively. The Td is an important factor which determines physical heat in the BF hearth. If the Td is too low, the heat carried by the molten iron cannot allow the blast furnace hearth to achieve good performance in desulfurization and slag tapping, which is detrimental to production. From the Td, it is proposed that the M/A of mixed burden cannot be less than 0.82. The softening characteristic temperatures of mixed burdens tended towards those of its corresponding high-basicity sinter and could be explained as followed. First, according to previous research on sinter mineralogy [17,29], increasing the MgO and Al 2 O 3 content facilitated the formation of magnesioferrite and dampened hematite obviously, whereas the amounts of calcium ferrite almost kept constant. The sinter containing higher amounts of MgO and Al 2 O 3 had a relatively lower reducibility. This fact resulted in earlier deformation, leading the lower T 10% and T 40% . Second, the increase in Al 2 O 3 content in sinter favored the formation of an Al 2 O 3 -rich primary slag phase with low melting temperature, such as the eutectic of 2CaO·SiO 2 -2CaO·Al 2 O 3 ·SiO 2 -FeO [22]. It has been suggested that MgO has a positive influence on softening temperatures [30]. However, the effect of Al 2 O 3 on softening was likely to be more dominant than MgO [17]. Thus, the softening temperatures were expected to decrease. It is indispensable to optimize the softening properties of mixed burden, but only when considering both the reduction property and gas permeability. In the softening process of the samples, its pressure drop was relatively low before intensely climbing. The wide softening zone benefited indirect reduction and decreasing coke rate. Therefore, it is believed that the softening property of S 2 P was the best where the certain M/A of the mixed burden was 0.82.

The Effects of MgO and Al 2 O 3 on Melting Properties
The effects of MgO and Al 2 O 3 content in sinter on the melting characteristic temperatures of mixed burdens are shown in Figure 4. With the increase in MgO and Al 2 O 3 content in sinter, the melting start temperature T m and the melting end temperature T f decreased from 1490 • C to 1436 • C and from 1558 • C to 1531 • C, respectively. The melting interval (T f -T m ) first increased slightly, and then increased greatly later. According to the experimental melting results, the lower edge of the cohesive zone and the corresponding T f moved upward and the thickness of melting zone and corresponding T f -T m increased. It should be noted that once the M/A of the mixed burden was less than 0.82, the melting properties were worsened significantly, which was not the expectation. The dripping temperature T d of S 1 P, S 2 P and S 3 P was 1534 • C, 1506 • C and 1481 • C, respectively. The T d is an important factor which determines physical heat in the BF hearth. If the T d is too low, the heat carried by the molten iron cannot allow the blast furnace hearth to achieve good performance in desulfurization and slag tapping, which is detrimental to production. From the T d , it is proposed that the M/A of mixed burden cannot be less than 0.82. Figure 5 shows   Tm and Td were related to the molten slag properties, such as liquidus temperature and viscosity. The slag was mainly derived from the gangue components in mixed burden and can be theoretically estimated with CaO-SiO2-MgO-Al2O3 system phase diagrams [31]. Figure 6 shows the phase diagrams of a CaO-SiO2-MgO-Al2O3 system, in which the fractions of Al2O3 are 5 mass%, 15 mass% and 20 mass%, respectively. From S1P to S3P, the liquidus temperature of the molten slag is obviously decreased. An aforementioned fact is that the low reducibility of sinter with high MgO and Al2O3 content can result in high    Tm and Td were related to the molten slag properties, such as liquidus temperature and viscosity. The slag was mainly derived from the gangue components in mixed burden and can be theoretically estimated with CaO-SiO2-MgO-Al2O3 system phase diagrams [31]. Figure 6 shows the phase diagrams of a CaO-SiO2-MgO-Al2O3 system, in which the fractions of Al2O3 are 5 mass%, 15 mass% and 20 mass%, respectively. From S1P to S3P, the liquidus temperature of the molten slag is obviously decreased. An aforementioned fact is that the low reducibility of sinter with high MgO and Al2O3 content can result in high FeO content remaining in the molten slag after mixed burden melting. FeO is a typical substance which can decrease the melting temperature of slag. In addition, it has been reported that blast furnace slag viscosity decreases with increasing FeO content [32]. Therefore, Tm and Td showed decreased trends. T m and T d were related to the molten slag properties, such as liquidus temperature and viscosity. The slag was mainly derived from the gangue components in mixed burden and can be theoretically estimated with CaO-SiO 2 -MgO-Al 2 O 3 system phase diagrams [31]. Figure 6 shows the phase diagrams of a CaO-SiO 2 -MgO-Al 2 O 3 system, in which the fractions of Al 2 O 3 are 5 mass%, 15 mass% and 20 mass%, respectively. From S 1 P to S 3 P, the liquidus temperature of the molten slag is obviously decreased. An aforementioned fact is that the low reducibility of sinter with high MgO and Al 2 O 3 content can result in high FeO content remaining in the molten slag after mixed burden melting. FeO is a typical substance which can decrease the melting temperature of slag. In addition, it has been reported that blast furnace slag viscosity decreases with increasing FeO content [32]. Therefore, T m and T d showed decreased trends.  Figure 7 shows photos of the residual materials and dripped materials after the experiments. The residual materials consisted of large amounts of slag phase and a relatively smaller amount of iron. In contrast, the dripped materials were mainly composed of iron and relatively less slag. To illustrate the roles of MgO and Al2O3 during melting, X-ray diffraction pa ens of the residual slags and dripped slags are shown in Figure 8. For the residual slags and dripped slags of S1P and S2P, the main minerals were melilite (Ca2(Al,Mg)[(Si,Al)SiO7]) and merwinite (3CaO·MgO·2SiO2). In contrast, the residual slag and dripped slag of S3P contained an additional component, monticellite (CaO·MgO·SiO2). The melting point of monticellite is 1390 °C, which is lower than those of melilite and merwinite. It is suggested that a higher MgO and Al2O3 content in sinter results in the generation of relatively lowmelting point components in the slag of mixed burden. It can be deduced that the mixed burden containing a high MgO and Al2O3 content with a low M/A can drip easily, resulting in the low Td and low ΔPmax. It is probable that the widest melting temperature interval of S3P where the M/A is less than 0.82 brings about the higher S value and worse permeability. Thus, under comprehensive consideration of the SM properties and the roles of MgO and Al2O3, the M/A of mixed burden made with high-basicity sinter and pellets should be about 0.82.   Figure 7 shows photos of the residual materials and dripped materials after the experiments. The residual materials consisted of large amounts of slag phase and a relatively smaller amount of iron. In contrast, the dripped materials were mainly composed of iron and relatively less slag. To illustrate the roles of MgO and Al2O3 during melting, X-ray diffraction pa ens of the residual slags and dripped slags are shown in Figure 8. For the residual slags and dripped slags of S1P and S2P, the main minerals were melilite (Ca2(Al,Mg)[(Si,Al)SiO7]) and merwinite (3CaO·MgO·2SiO2). In contrast, the residual slag and dripped slag of S3P contained an additional component, monticellite (CaO·MgO·SiO2). The melting point of monticellite is 1390 °C, which is lower than those of melilite and merwinite. It is suggested that a higher MgO and Al2O3 content in sinter results in the generation of relatively lowmelting point components in the slag of mixed burden. It can be deduced that the mixed burden containing a high MgO and Al2O3 content with a low M/A can drip easily, resulting in the low Td and low ΔPmax. It is probable that the widest melting temperature interval of S3P where the M/A is less than 0.82 brings about the higher S value and worse permeability. Thus, under comprehensive consideration of the SM properties and the roles of MgO and Al2O3, the M/A of mixed burden made with high-basicity sinter and pellets should be about 0.82. To illustrate the roles of MgO and Al 2 O 3 during melting, X-ray diffraction pattens of the residual slags and dripped slags are shown in Figure 8. For the residual slags and dripped slags of S 1 P and S 2 P, the main minerals were melilite (Ca 2 (Al,Mg)[(Si,Al)SiO 7 ]) and merwinite (3CaO·MgO·2SiO 2 ). In contrast, the residual slag and dripped slag of S 3 P contained an additional component, monticellite (CaO·MgO·SiO 2 ). The melting point of monticellite is 1390 • C, which is lower than those of melilite and merwinite. It is suggested that a higher MgO and Al 2 O 3 content in sinter results in the generation of relatively lowmelting point components in the slag of mixed burden. It can be deduced that the mixed burden containing a high MgO and Al 2 O 3 content with a low M/A can drip easily, resulting in the low T d and low ∆P max . It is probable that the widest melting temperature interval of S 3 P where the M/A is less than 0.82 brings about the higher S value and worse permeability.

Conclusions
The softening-melting properties of mixed ferrous burden made from high-basicity sinter with increased MgO and Al2O3 content and acid pellets and the roles of MgO and Al2O3 during melting were investigated. Several conclusions were obtained.
(1) With decreasing MgO mass%/Al2O3 mass% in mixed burden, the softening-melting characteristic temperatures of mixed burdens tended to decrease. The softening interval widened slightly at first, and then narrowed, while the melting interval first increased slightly, and then increased greatly. The permeability of the melting zone deteriorated once the MgO mass%/Al2O3 mass% was less than a certain content. (2) The low melting temperature and viscosity of the slag of mixed ferrous burden with a low MgO mass%/Al2O3 mass% was the main reason for the reduction in softeningmelting characteristics. (3) On the whole, with decreasing MgO mass%/Al2O3 mass% in mixed burden, the softening properties were improved whereas the melting properties were worsened. Considering the iron ore reduction and thermal state of blast furnace hearth, mixed burden had the optimal softening-melting properties at an MgO mass%/Al2O3 mass% of 0.82.
Author Contributions: Z.L. and T.L. performed the experiments, material characterization, and paper writing; C.S. and S.Y. contributed to the data analysis and design of the experiment. Q.W. contributed to the conceptualization. All authors have read and agreed to the published version of the manuscript.
Funding: This work was financially supported by the National Natural Science Foundation of China (No. 52174319) and Department of Education of Liaoning Province (LJKQZ20222303).

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.

Conclusions
The softening-melting properties of mixed ferrous burden made from high-basicity sinter with increased MgO and Al 2 O 3 content and acid pellets and the roles of MgO and Al 2 O 3 during melting were investigated. Several conclusions were obtained.
(1) With decreasing MgO mass%/Al 2 O 3 mass% in mixed burden, the softening-melting characteristic temperatures of mixed burdens tended to decrease. The softening interval widened slightly at first, and then narrowed, while the melting interval first increased slightly, and then increased greatly. The permeability of the melting zone deteriorated once the MgO mass%/Al 2 O 3 mass% was less than a certain content. (2) The low melting temperature and viscosity of the slag of mixed ferrous burden with a low MgO mass%/Al 2 O 3 mass% was the main reason for the reduction in softening-melting characteristics. (3) On the whole, with decreasing MgO mass%/Al 2 O 3 mass% in mixed burden, the softening properties were improved whereas the melting properties were worsened. Considering the iron ore reduction and thermal state of blast furnace hearth, mixed burden had the optimal softening-melting properties at an MgO mass%/Al 2 O 3 mass% of 0.82.
Author Contributions: Z.L. and T.L. performed the experiments, material characterization, and paper writing; C.S. and S.Y. contributed to the data analysis and design of the experiment. Q.W. contributed to the conceptualization. All authors have read and agreed to the published version of the manuscript.
Funding: This work was financially supported by the National Natural Science Foundation of China (No. 52174319) and Department of Education of Liaoning Province (LJKQZ20222303).

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.